1. Field of the Invention
This invention relates to nitride semiconductor substrates and a method of producing a nitride semiconductor substrates. InGaN-type blue light emitting devices (LEDs+LDs) are produced by piling n-type or p-type GaN (gallium nitride), InGaN (indiumgallium nitride), AlGaN (aluminumgallium nitride) films on sapphire (Al2O3) substrates or silicon carbide (SiC) substrates in vapor phase. Almost all of the blue light emitting diodes (LEDs) and blue light laser diodes (LDs) are made on sapphire substrates at present. Sapphire substrates have advantages of an established production method, a stable supply, low cost, affluent accomplishment, long lifetimes and high reliability.
This application claims the priority of Japanese Patent Application No. 2003-345910 filed on Oct. 3, 2003, which is incorporated herein by reference.
Sapphire has a lattice constant different from the lattice constant of GaN or InGaN. GaN or InGaN films grown on sapphire have high density of dislocations. Despite the difference of lattice constants, sapphire enables device makers to produce light emitting diodes (LEDs) which produces light of a wide range of wavelengths from blue to green. GaN blue light emitting diodes on the sapphire substrate are utilized widely. InGaN/sapphire LED are enough for cheap LEDs of a wide range of wavelengths.
Sapphire substrates have a variety of drawbacks, e.g., non-cleavage, insulation and lattice-misfit. Lack of cleavage prevents makers from producing neat resonator mirrors of on-sapphire laser diodes (LDs) by natural cleavage. The resonator mirrors should be made to be flat by mechanical dicing and polishing, which raises cost and lowers yield.
Sapphire is an insulator. Insulation prevents an on-sapphire device from having an n-electrode on the bottom. Instead of the bottom, an n-electrode is formed on an n-type thick GaN film by piling the thick GaN film on the sapphire substrate, growing other n- or p-GaN, InGaN layers, etching a part of the piled layers from the top till the thick n-GaN film. The top n-electrode is a drawback of the on-sapphire LEDs. Top n-electrode and p-electrode are wirebonded to a n-leadpin and a p-leadpin respectively. The top n-electrode doubles the times of wirebonding. The top n-electrode induces another inconvenience. The top n-electrode on the thick n-GaN film requires an extra area in an LED, which decreases the ratio of emission area to the whole LED area. These are drawbacks caused by insulator sapphire.
Another drawback is the lattice misfit between sapphire and film nitride (GaN, InGaN, AlGaN). GaN, InGaN or other nitride films grown on sapphire substrates include dislocations of high density from 109 cm−2 to 1010 cm−2. High dislocation density matters little in the case of LEDs which emit low power light with low driving current density. In future, high power LEDs will be required. The high dislocation density may be an obstacle in future high power LEDs which are made upon sapphire substrates.
GaN substrates are desired for making LEDs or LDs instead of sapphire substrates. If a GaN substrate were used, good GaN, InGaN films would be grown on the GaN substrate for the sake of lattice-fitting. An n-type GaN is not insulating but conductive. An n-electrode could be formed on the bottom of an n-GaN substrate. The n-electrode would come into contact with a cathode pin only by die-bonding an LED chip on the cathode pin. The difficulty of the top n-electrode would be solved. Elimination of top n-electrode could widen an effective area. Use of the facet-growth method, which is described later, will reduce dislocation density down to 107 cm−2 to 108 cm−2. An epitaxial GaN wafers can be easily and exactly divided into individual chips by natural cleavage.
However it is difficult to make a large GaN single crystal. When solid GaN is heated, solid GaN is not melted but sublimes into vapor GaN. Liquid phase crystal growth (Czochralski, Bridgman) is forbidden. Then vapor phase epitaxy, which was inherently a method of making thin films of GaN, is diverted to making thick GaN substrate. The vapor phase method prepares an undersubstrate, piling a GaN film on the undersubstrate for long hours, eliminates the undersubstrate and obtains a freestanding GaN substrate.
Assuming that a GaN freestanding substrate was obtained, a good low-dislocation density GaN film could be epitaxially grown on the GaN substrate due to lattice-fitting. But the fact is otherwise. High density of dislocations accompany GaN films grown on the GaN substrate in fact. Why the epi-films are plagued with high density of dislocations? The GaN substrate has inherent high density of dislocations. The dislocations survive also in the epi-films. The epi-films transcribe all the dislocations on the GaN substrate. Dislocations are transferred from the substrate to the films. Since the GaN substrate has high density of dislocations, the epi-layers have also high density of dislocations. The use of a GaN substrate cannot reduce dislocations on epi-layers grown thereon. The inventors have been aware of the facts.
2. Description of Related Art
When GaN films are grown directly upon a sapphire substrate with maintaining a C-plane surface, the grown GaN are plagued at plenty of dislocations. The inventors of the present invention have contrived a novel (face growth method) method for reducing dislocations in films by making facet pits on the growing GaN surface, maintaining the facets forming the pits, growing the GaN film with dislocations, sweeping the dislocations into bottoms of the facet pits, producing dislocation bundles at the bottoms of the pits and decreasing dislocations in other regions. Japanese Patent Laying Open No. 2001-102307 proposed the face growth method. The proposed method enables the inventors to make low dislocation density GaN freestanding substrate of low dislocation density of 107 cm−2 to 108 cm−2 by counting a bundle of dislocations as a single dislocation. However dislocations are not annihilated in the new method. Since many dislocations are gathered and unified into a bundle, the number of dislocations seems to decrease. However, the apparent decline of dislocations is meaningful and the facet-grown GaN substrates are useful, because good devices can be fabricated upon lower dislocation density parts.
The freestanding crystal of GaN is not all the necessary conditions of a wafer for making devices thereupon. Patterns are formed on semiconductor wafers by photolithography. Flatness or immunity of distortion is necessary. Furthermore, the surfaces should be extra smooth, mirror-surfaces. Here stepping aside the distortion, the outstanding problem is the smoothness of surfaces of GaN wafers. The smoothness cannot be expressed directly. Smoothness is an antinomy of ruggedness. The ruggedness can be represented by surface roughness.
Superficial smoothness is a problem different from the problem of dislocations. Polishing would raise smoothness of substrates. Repetitions of polishing would further enhance the smoothness of the substrates. However repetitions of polishing raises cost of substrate wafers. The scope of the desirable smoothness should be determined by reconciling the device-desired properties with the cost.
No GaN substrate of a practical scale had existed till recently. Few documents referred to surface roughness of GaN substrates. Japanese Patent Laying Open No. 10-256662, “Method of producing nitride semiconductor substrates and method of producing nitride semiconductor devices”, says that height differences on surfaces of GaN substrates should be less than ±1 μm. An n-GaN layer, n-AlGaN/GaN superlattice cladding layers, an InGaN active layer, p-AlGaN/GaN superlattice cladding layers, a p-GaN capping layer would be grown in turn on a substrate. The first n-GaN layer has an about 5 μm thickness. Other layers are all very thin. The n-AlGaN layers in the superlattice are 0.02 μm thick. The GaN layers in the same superlattice are 0.02 μm thick. The InGaN active layer is 0.03 μm thick. If the substrate had surface roughness over ±1 μm, excess height differences would appear on the upper AlGaN, GaN and InGaN layers epitaxially grown on the over ±1 μm rugged substrates and the excess height differences would degrade the properties of blue light emitting diodes. Then Japanese Patent Laying Open No. 10-256662 insisted upon under ±1 μm roughness of GaN substrates.
The 5 μm n-GaN layer would have alleviated the bad influence of the rugged substrates. Thus the lowest limit of height differences should be ±0.3 μm in Japanese Patent Laying Open No. 10-256662.
Irrespective of the dislocation density, Japanese Patent laying Open No. 10-256662 proposed a preferable scope of roughness from ±1 μm to ±0.3 μm for the sake of preventing the films grown on the substrates from being torn out. Since the scope of preferable roughness is represented with a symbol ±, the scope should be doubled. The desired hill/valley differences should be 2 μm to 0.6 μm in Japanese Patent laying Open No. 10-256662. The hill/valley differences should be identical to be an average of differences between neighboring hills and valleys. Thus the hill/valley differences correspond to twice of Ra (=2 Ra). Then the hill/valley differences of ±1 μm to ±0.3 μm would be equal to Ra2 μm to Ra0.6 μm.
Ra and Rms are both roughness parameters. Values of Ra and Rms depend upon morphology of object surfaces. The relation between Ra and Rms is not simple. The rate of Ra to Rms is not constant. Without exception, Ra is smaller than Rms. When the surface disturbance is regular and the rate Rms/Ra is smallest, the Rms/Ra rate is about Rms/Ra=1.3. In the cases of irregular roughness, the rate Rms/Ra takes values of 1.3 to 2. In an average case, the rate Rms/Ra may be 1.5. Then Ra=0.3 μm to 1 μm of Japanese Patent Laying Open No. 10-256662 should correspond to Rms=3 μm to 1 μm.
Japanese Patent Laying Open No. 10-256662 asserted that the surface of the GaN substrates should be smaller than ±1 μm. The document alleged that roughness over ±1 μm should deteriorate the properties of LEDs fabricated upon the GaN substrates. But the document mentioned nothing about grinding, lapping and polishing of GaN substrates. Japanese Patent Laying Open No. 10-256662 was indifferent to dislocation density on GaN substrates and paid no attention to the influences caused by the dislocation density upon the films grown thereon.
M, Ishida, M. Ogawa. K. Orita, O. Imafuji, M. Yuri, T. Sugino, K. Itoh, “Drastic reduction of threading dislocation in GaN regrown on grooved stripe structure”, Journal of Crystal Growth 221(2000) p 345-349 pointed out a drawback of the ELO, which makes a SiO2 (or SiN) ELO mask with many windows on a sapphire substrate and grows GaN films on the ELO-masked sapphire. The existence of the foreign material of the mask would induce cavities and non-uniform parts in the GaN film. For suppressing the cavities and the non-uniformity, Ishida et al. formed a plenty of 1 μm deep grooves by reactive ion-etching, which have a 7 μm wide bottom and 45 degree inclining (1-102) side walls and extend in parallel to <11-20> direction in the GaN film which had been grown along the c-axis. Then Ishida regrew a 2 μm thick GaN film of the grooved GaN film. The GaN film turned out to be of low dislocation density.
Ishida alleged that the c-axis growing speed and the <1-101> direction growing speed could be controlled by varying the group 5/3 mol rate of material gases (the ratio of ammonia/trimethylgallium). When the 5/3 ratio is 5500 times, the dislocations would converge at center lines on the surface of a 2 μm thick GaN and the dislocation density would substantially reduce. The group 5 gas is ammonia gas. The group 3 gas is trimethylgallium (TMG; (CH3)3Ga) in Ishida. Group 5/3 mol ratio means a quotient of mols of an NH3 gas supply divided by mols of a TMG gas supply. That the group ratio 5/3 is 5500 means that 5500 mols of ammonia is supplied to every 1 mol of TMG. Supply of group 5 gas is extraordinarily large in comparison with group 3 gas (TMG). The excess large 5/3 rate was inherent to Ishida's new method. Ordinary MOCVDs take a 5/3 rate between 1000 and 3000. Ishida supplied far excess ammonia gas for controlling the on-bottom vertical growing speed and the on-slant slanting growing speed.
Ishida's report was difficult to understand. FIG. 6 shows a section of a groove prepared by Ishida in a growing GaN film. Ishida is explained on FIG. 6. Horizontal sides are C-planes (0001). A bottom of a groove is a C-plane (0001). The C-plane is a Ga-plane on which gallium atoms align overall. Two side slanting walls are (1-101) planes. The (1-101) planes are N-plane on which nitrogen atoms align overall. The slanting growing speed on (1-101) is denoted by “u”. The vertical growing speed on (0001) is denoted by “v”. The higher the 5/3 rate raised, the faster the <1-101> direction (on-N-plane) growth (u) was accelerated. The lower the 5/3 rate fell, the faster the <0001> (on-Ga-plane) direction was accelerated, Ishida alleged. An upper width of the groove is denoted by “M” (e.g., 7 μm). A depth of the groove is denoted by “F” (e.g., 1 μm). A depth of GaN recrystallization is denoted by “W” (e.g., 2 μm).
In FIG. 6, slanting growth (speed=u) starts from the slanting walls (point P) of the grooves and vertical regrowth (speed=v) starts from the flat sides of the groove at the same moment. If when the slanting (speed=u) growth from point P attains at point O, the vertical regrowth accidentally has a thickness W, dislocations on the groove would converge at middle point O. An assemble of dislocations at point O is count as one dislocation, because the dislocations are converges at the spot O. Thus dislocations seems to be reduced. Ishida's contrivance was complicated and difficult to accomplish. The condition is too rigorous. Since the regrowth height is W, the distance from the bottom to the regrowth surface is (F+W), where F is the groove depth. For simplicity, P is a middle of the slant. The distance from slant point P to the middle point O is [{W+(F/2)}2+{(M/2)−(F/2)cotΘ}2]1/2. Here Θ is an angle of the slant to the horizontal plane. Crystal growth starting from P proceeds in the direction vertical to the slanting wall. A horizontal component of the distance OP is (M/2)−(F/2)cotΘ. A vertical component of the distance OP is (F/2)+W. The rate the horizontal distance to the vertical distance should be equal to tanΘ for coinciding the regrowth attaining to point O with the slanting growth attaining to the same point O.tanΘ={(M/2)−(F cot Θ/2)}/{(F/2)+W}.  (1)
Here Θ=45 degrees by Ishida. Thus Eq.(1) should be 1.
The vertical regrowth speed is v and the vertical regrowth height is W. When the thin film thickness attains to W, the slanting growth(speed=u) from P should reach point O. It takes time of [{W+(F/2)}2+{(M/2)−(F/2)cot Θ}2]1/2/u for the slanting $$ growth to progress from P to O. It takes time of W/v for the vertical regrowth (speed=v) to raise from the flat sides to the regrowth thickness. The speed rate u/v can be raised by enhancing the 5/3 rate. Namely u and v are controllable variables. If the speed rate u/v is given to satisfy the following relation,[{(F/2)+W}2+{(M/2)−(F cot Θ/2)}2]1/2/W=u/v,  (2)
Dislocations on the groove would be gathered at point O by the regrowth.
If the materials are supplied at a rate at which u/v satisfies Eq. (2), recrystallization of a thickness W can assemble dislocations on the groove at middle point O just above the groove. An assembly of dislocations gathering at a point is counted as one dislocation. Thus Ishida can reduce dislocation density down to 1/100 in the most fortunate case. Ishida alleged that when the group 5/3 rate was 5500, the ratio u/v satisfied Eq. (2) and dislocation density was reduced. In an example of Ishida, F=1 μm, W=2 μm, M=7 μm and Θ=45°, Eq.(2) becomes,[{(F/2)+W}2+{(M/2)−(F cot Θ/2)}2]1/2/W=2.  (3)
Thus supply of material gases at the group 5/3 gas rate determined by u/v=2 enables just 2 μm thick recrystallization to reduce dislocations. Ishida alleged that 2 μm of recrystallization on the grooves succeeded in reducing dislocation density in the example by gathering dislocations at point O, which is a cross point of the recrystallization level with a normal standing at a middle point of the bottom of the groove.
Ishida's prior art reference was an improvement of film growth for replacing the epitaxial lateral overgrowth (ELO). Ishida et al. aimed at producing not a thick GaN substrate but a GaN thin film of a 2 μm to 3 μm thickness. The present invention differs from Ishida in production of thick substrates or thin films and in objects. Ishida is now cited since the idea of reducing dislocations by grooves is intriguing.
However, Ishida's method is applicable only on rigorously restricted conditions of the predetermination of the groove depth F, the groove width M, the regrowth thickness W and the definite value of the u/v rate. If the recrystallization thickness is over the predetermined W or is under W, the dislocation density does not decrease, because dislocations diffuse out of the dislocation bundles again. All the grooves should take the predesigned values of depth F, width M and inclination Θ. All the grooves should align in parallel to <11-20>.
Ishida said that the inclination Θ should be Θ=45 degrees. A (1-102) plane crosses at 43.19 degrees to the C-plane (0001). Perhaps Ishida aimed at the slants of the grooves should coincide with (1-102) planes.
Ishida's proposal is an interesting method based upon the MOCVD. Ordinary MOCVDs supply group 3 (trimethylgallium etc.) and group 5 (ammonia) material gases at a group 5/3 rate of 1000 to 3000. But Ishida supplied the groups 3 and 5 material gases at a higher 5/3 rate=5500. Ishida's method increases the ammonia loss, which raises the cost of material gases. The inventors of the present invention think that the prevalent MOCVD methods are unsuitable, since carbon atoms contaminate the object nitride semiconductor substrates.
For the above reason, the inventors of the present invention produce GaN films by either a hydride vapor phase epitaxy (HVPE) method or a metallorganic chloride method (MOC) instead of the dominant MOCVD method. Both the HVPE and the MOC have an advantage of being immune from carbon contamination.
Instead of the Ga organic metals (trimethylgallium, triethylgallium), the HVPE employs a Ga metal melt in a Ga boat furnished at an upper region of a hot-wall type furnace as a Ga source. The Ga metal is heated by heaters and in converted into a Ga metal melt. The HVPE synthesizes once an intermediate compound of gallium chloride (GaCl) by blowing a mixture of hydrogen gas (H2) and hydrochloride gas (HCl) to the molten Ga in the Ga boat. The HVPE further synthesizes gallium nitride (GaN) by blowing a mixture of hydrogen gas (H2) and ammonia (NH3) gas to a heated undersubstrate sustained on a susceptor and piles GaN crystal on the heated undersubstrate. The HVPE is immune from carbon contamination, since the materials contain no carbon.
The MOC method employs an organic metal (trimethylgallium, triethylgallium) as a Ga material. Unlike the MOCVD, the MOC once produces GaCl by a reaction of HCl with the organic metal at a first step. The MOC produces GaN by another reaction of GaCl with NH3 at a second step and piles GaN on an undersubstrate installed at a lower region of a furnace. The undersubstrate has no chance of being in contact with the organic metal. Since GaCl is once produced, the MOC can suppress carbon contamination.
Japanese Patent Laying Open No. 2000-12900, “GaN single crystal substrate and method of same”, which was proposed by the present inventions, disclosed a production method of a GaN substrate in vapor phase. The present invention employs an as-cut GaN substrate made by the method of Japanese Patent Laying Open No. 2000-12900 as a starting GaN substrate.